Solid Lubricants as Friction Modifi ers 179
the graphite likely reduces the bonding energy between the hexagonal planes of the graphite to a
level that is lower than the adhesion energy between a substrate and the graphite crystal. This allows
for lamellar displacement of the graphite crystals when shear forces are applied to the graphite fi lm.
The result is a reduction of friction and corresponding lubrication. Because water vapor is a require-
ment for lubrication, graphite is usually not effective as a lubricant in a vacuum atmosphere.
The lubricating ability of graphite as a function of temperature is very good. Graphite is
able to withstand continuous temperatures of up to 450°C in an oxidizing atmosphere and still
provide effective lubrication. The oxidation stability of graphite depends on the quality of the
graphite, the particle size, and the presence of any contaminants that might accelerate the oxi-
dation. Graphite will also function at much higher temperatures on an intermittent basis. Peak
oxidation temperatures are typically near 675°C. For these instances, modifying the composition
of the graphite mixture may be necessary as a way to control its rate of oxidation.
The thermal conductivity of graphite is generally low. For example, primary-grade synthetic
graphite has a conductivity of ~1.3 W/mK at 40°C. Amorphous graphite is even less conducting and
is sometimes considered for providing some degree of thermal insulation for specifi c applications.
6.2.2 MOLYBDENUM DISULFIDE
Molybdenum disulfi de is the second signifi cant solid lubricant widely used in industry. It has been
used since the early nineteenth century for lubrication applications. MoS 2 , also known as molybde-
nite, is a mined material found in thin veins within granite. Lubricating-grade MoS 2 is h igh ly refi ned
by various methods to achieve a purity suitable for lubricants [6]. This purity usually exceeds 98%.
MoS 2 is commercially available in a variety of particle size ranges. Table 6.4 lists basic properties
for molybdenum disulfi de. The low friction of MoS 2 is an intrinsic property related to its crystal
structure, whereas graphite requires the adsorption of water to behave as an effective lubricant.
Molybdenum disulfi de achieves its lubricating ability with a mechanism similar to graphite. Just
like graphite, MoS 2 has a hexagonal crystal lattice structure.
Sandwiches of planar hexagonal Mo atoms are interspersed between two layers of sulfur
atoms. Similar to graphite, the bond strength between the hexagonal planes between the sulfur
atoms are weak van der Waal-type bonds when compared to the strong covalent bond between
molybdenum and sulfur atoms within the hexagonal crystal. Orientation of the MoS 2 crystallites
is important if effective friction reduction is to be achieved. MoS 2 has anisotropic properties that
are comparable to graphite. When a force is applied parallel along the hexagonal planes, the weak
bond strengths between the planes allow for easy shearing of the crystal, resulting in a lamellar
TABLE 6.4
Characteristics of Hexagonal Molybdenum
Disulfi de
Property Value
Bulk hardness 1.0–1.5 Ʊ
Coeffi cient of friction 0.10–0.15
Color Blue-gray to black
Electrical conductivity Semiconductor
Luster Metallic
Melting point >1800°C
Molecular weight 160.08
Service temperature Up to 700°F
Specifi c gravity 4.80–5.0
Thermal conductivity 0.13 W/mK at 40°C